Semiconductor devices, for example semiconductor lasers and photodetectors, find many applications for example in the fields of optical data storage, printing and communications. Such wide spread use of semiconductor devices has lead to great demand for semiconductor devices fabrication techniques which are suitable for producing high performance devices in large volumes and at low cost.
A key element which determines the performance of a semiconductor laser is whether or not its output facets are coated and for a photodetector whether or not its input facets are coated. An uncoated semiconductor laser facet will typically have a reflectivity of approximately 30%. This reflectivity is often not optimum for the performance of the laser. The facets of semiconductor lasers are thus often coated with for example a high reflectivity (HR) coating, or a low or anti-reflectivity (AR) coating, or both. DFB (Distributed Feed Back) lasers, particularly if a phase shift in the grating is incorporated, require that both facets are coated with an AR coating in order to suppress Fabry-Perot modes and to ensure lasing at the grating determined wavelength. High power semiconductor lasers conversely are often coated at one facet with an HR coating and at the other facet with an AR coating. Facet coating semiconductor lasers can lead to better high temperature performance, lower threshold currents, and more efficient operation.
Despite these advantages facet coating is rarely used for example in the telecommunications industry for higher volume, lower cost semiconductor lasers due to the increase in fabrication costs caused by the facet coating processes. Semiconductor lasers are grown on a wafer by depositing a number of semiconductor layers, and etching a structure into these layers, then coating the upper surface of the wafer with dielectric and metal layers to allow the lasers to be electrically contacted. Conventionally, the wafer of semiconductor lasers is then cleaved to form many fragile bars of semiconductor lasers, and these bars are held together to expose the laser facets which are coated with a facet coating to alter their facet reflectivity. Care must be taken to shield the upper and lower surfaces of the lasers so as to avoid the facet coating material being deposited on these surfaces and impeding electrical contacts with the lasers. This prior art technique for facet coating thus involves the handling and accurate alignment of many fragile semiconductor bars and consequently increases fabrication costs considerably and reduces fabrication yields.
U.S. Pat. No. 5,185,290 discloses a method in which conventionally formed semiconductor lasers are etched, while still on the wafer, to form a laser facet and these lasers facets are subsequently coated. While this technique addresses some of the problems of facet coating lasers, it remains a complex and expensive fabrication technique.